Riddle me this.. How much backing to Braid??

[planter]

I sure can't figure it out but I'm NOT very bright. The reel is a 3500B that can hold 305 yards of 8lb line.

Problem is I want to back up 20lb Spiderwire with 17lb big game. How could I ever know how much of the backing to put on to use up the entire spool (150yds) of braid on top of it.
$35.00 bucks each for two new Baitrunner spools. Could have got them a little cheaper on line but I Petes not in business I won't have a place to yak and but chunking meat...